outboard size for a sailboat

[parito]

We own a 21 foot twin keel sailboat.

With 6hp honda outboard, and after scrubing the hull from barnicles we are able to reach ~5.4 knots. Our hull speed based on a formula is 5.9kt.

We are pondering if lower pitch propeller would help achieve hull speed based on assumption that a typical prop is designed for inflatables mostly .. (we currently have 9-1/2 x 8-5/8 inches prop). That additional power is really needed when you are beating into the steep waves of Thames Estuary..

If you use outboard as your primary engine, could you also post your:

LWL / outboard / avg max water speed?
I'll start with mine

19ft / honda 4stroke 6hp / 5.4kt

 

[VicS]

19ft / Evinrude 2 stroke 6hp Yachtwin / no idea kt


6 hp ought the be adequate for 21ft., but might consider 8hp if no significant weight penalty in the model range chosen

A "sail" prop, ie smaller pitch than standard perhaps with larger diameter and/or blade area, in theory ought be better for a displacement hull..... the down side is that the engine revs will be higher for the same speed though the water but at least that means it is producing more power.
My prop is 9¼" x 6½" and has big "mickey mouse ear" blades. The standard prop for the 6 hp 'rude was 8½" x 9".

My engine has a long shaft but is designed, with modified exhaust outlets, to be mounted at the same height as a standard shaft engine. This puts the prop 5" deeper, so that it does not surface when the boat pitches, without increasing the back pressure on the exhaust. It also prevents gases being drawn into the prop when going astern so thrust is not diminished

 

[Seajet]

I get on fine with a 5hp 2 stroke Mariner outboard - standard shaft engine in a cockpit well, 22' boat weighing probably 3000 lbs and a slippery single lift keel hull shape and low windage ) but had to slightly trim the prop blades to achieve max revs and just over 5 knots.

 

[Rigger Mortice]

Any formula to work out speed is, at best, a fair estimate rather than a hard and fast rule. 5.4 knots certainly isn't bad.

A new prop may improve things. It may not. The trouble is it will cost you the best part of £100 to find out.

It's worth trying a rev counter on your present setup to see exactly what is happening. When you have done that try giving Steel Developments a call. They're on the web. I got some very good advice from them several years ago when I had a similar dilema.

 

[Lakesailor]

What sort of throttle opening do you have when achieving the max speed?
It's a case of diminishing returns on displacement hulls.
I've had three 19 footers and a 15 ft motor boat all using 4 or 5hp motors.

Generally up to 4/5 knots has been a slightly open throttle. Getting up to 5.5 knots has been, possibly, half throttle. Beyond that it's all noise and vibration, but no more speed.


I happen to know that in the 1st part of this Video I am towing my motor boat(15ft) using Feckless (17ft, 4hp Mariner)) at 3.9 knots. In the 2nd part of the vid I am just touching 5 knots. It wouldn't go any faster if I opened the throttle more.
(handheld GPS, no current on Windermere)

Don't know if that helps.

 

[parito]

We did see our boat sail > 5.9 knots in ideal conditions, so I believe hull speed formula is about right..

On our recent trip across east/south coasts we really missed the power - we were able to only do 4 knots max. Then we found our engine has throttle limiter. We removed it - same results. Only 4 knots. then we scrubbed barnacles - up to 5.5 knots. However I am still very interested in changing the prop to allow engine reach max rpms. As the previous poster suggested I will test rpms with rpm reader first, to make sure we are not reaching those already, but I am very confident we are not - you can actually hear motor can reach higher rpms on idle compared to when you are in gear.. And it makes sense - displacement hull is much harder to move, so if the propeller pitch is too big, lowering the pitch should help. Maybe it's just one of those things sailors never bother to check ?

People say 5.4 knots is plenty, but if you happen to motor into steep waves and strong headwind I think you need every horse.. Plus it's healthier for your outboard..

Again this is just a hypothesis for now, as it might turn out we already have the lowest pitch prop possible for honda outboard

 

[Rigger Mortice]

If you've removed a throttle limiter it may be worth checking whether or not you should change the jets in your carb. There may be a bit more power to squeeze out of your engine.

 

[Daydream believer]

Does a lot depend on power curve?
Someone will know i am sure
Is not best throttle opening the one where the revs are just on the max torque of the engine
On some cars ( for instance) i seem to recall max torque at 5000 rpm but max allowable rpm 6000
Is there any point trying to go to max revs if the power drops off & you cannot actually do them

 

[Croak]

6hp 24foot .....................7.5mph














Oh, it's a cat.

 

[lw395]

The 'hull speed' is not a brick wall.
Depending on the hull shape it can be anything from a point where the power/speed curve has a kink in it through to an arbitrary slope of that curve.
The prop that gives you most speed may not give you much acceleration or may be useless for driving into a chop.

FWIW, Yamaha 2hp, standard prop, about 5knots (eventually), 20LOA displacement about 1 ton?
That was the normal prop for a tender, full throttle, the revs built up as the boat gathered speed, but probably nowhere near max power of the engine.
A 3.5 Malta will tow another boat of the same class against the tide in Portsmouth Harbour entrance. Must be 4 knots or so to make ground even keeping out of the deep water as much as possible.

 

[parito]

FWIW, Yamaha 2hp, standard prop, about 5knots (eventually), 20LOA displacement about 1 ton?

That's why I am curious - 2hp, 1 ton displacement, 5 knots.

Ours - 6hp, ~1300kg, 5.4 knots.

Surely this has something to do with a prop?

 

[Seajet]

That's why I am curious - 2hp, 1 ton displacement, 5 knots.

Ours - 6hp, ~1300kg, 5.4 knots.

Surely this has something to do with a prop?

Parito,

it could also be likely to do with draggy early generation twin keels, windage.

5.4 knots sounds OK anyway, are you a descendant of Donald Campbell ?!

Waterline length - not overall on deck - X 1.4 the square root of the WL decrees max speed, unless the boat can plane or surf.

A 6hp engine should be fine as an auxilliary - not motoring into gales but an assistance - so maybe you're expecting too much in strong weather, or the bottom is draggy or the prop isn't allowing full revs, but also you're expecting peak idealistic speed which not many of us get, what sort of calibrated instrument are you checking the speed with ?

 

[C08]

I think hull speed is not the important thing here for a small cruising yacht although I do understand your wish to fully understand what is going on. It is the ability to maintain (say) 5knots even with a bit of wind on the nose or adverse current that will enable you to get to your destination without taking excessive time. taking too long often means missing tidal windows for entering harbours, more time for the weather to turn nasty etc apart from more wear and tear on the crew. Many sailboats engines are deliberately overpropped in the interest of more restful motoring (noise) and perhaps economy. Yes your engine is probably overpropped and you need a rev counter to sort that out but an 8HP engine would give you that bit of extra power to push a tide or headwind, without being excessively heavy in weight or on fuel.

 

[lw395]

.....

Waterline length - not overall on deck - X 1.4 the square root of the WL decrees max speed, unless the boat can plane or surf.

.....

It roughly defines the point at which a boat starts to expend significant power climbing its own bow wave. But some hull forms with fine bows can exceed '1.4' without planning or surfing.
On other hull forms it is a point at which the power needed rises very very steeply.
It more of a guideline that a fundamental thing like the speed of light.
In fact, 150 years ago, the multiplier was taken to be 1.1 by many people, design has improved.

 

[parito]

5.4 knots sounds OK anyway, are you a descedant of Donald Campbell ?!

that was funny although I have to admit I had to look it up

Re bottom: as posted in my second post - we did scrub the bottom which improved things from 4 to 5.4 knots.

Yes, windage, draggy keels, all that affects things - what I am surprised is no one can confirm or deny my hypothesis that:

"in general, all other things being equal, higher displacement vessel (sailboat) will benefit from a lower pitch propeller compared to a standard one. This is based on the assumption that a standard propeller is chosen by a manufacturer optimized for an inflatable, not a 1.3ton sailboat. Being a heavier vehicle to push, the engine is not able to reach it's optimum RPMs to produce it's maximum power and work in an optimum RPM range"

I just bought tachometer from ebay - hope to measure rpm's over the weekend..

 

[Tranona]

You are correct with your basic proposition, which is why some manufacturers offer a finer pitch "sail" prop. Ask Honda what their recommendation is for your setup.

However you are unlikely to get a significant increase in maximum speed, but what you may get is what you are looking for which is the ability to maintain speed in adverse conditions. As others have said, 6hp is enough, but an outboard is always going to be less than ideal for the job because you can't swing a big enough diameter prop and it is positioned in a less than ideal place for efficient propulsion. An inboard of similar power with a reduction box would probably turn a 12" prop. I used to have a 9hp Yanmar in a boat weighing nearly 3 times more than yours and got similar speed to you - but with a big reduction ratio and a 15" prop.

 

[William_H]

I have a 6HP 2 stroke like Vic on a 21fter. I have never felt the need for more power indeed I think I could go a lot less for a lighter motor. However other things are significant like 2 cylinders means less noise and vibration which is very desirable for long motoring runs. I use a standar pitch more designed for dinghies than keel boats. This means that the motor may not reach max revs but does meen less RPM at gentle cruise so again quieter. I don't have tides to beat against so I am usually happy at 4knots and low throttle setting. However with this motor I once towed 3 other 24fters in a line at a respectable 3 knots in dead calm conditions.
So much depends on your priorities I would always I hope be sailing in strong head winds so I don't need lots of power. I just use it for the odd traverse under bridges and dead calms. An adjustable height o/b mount is very important for moptring in rough water. good luck olewill

 

[John the kiwi]

I have a 26 ft light displacement trailer -sailor.
This has a planing hull and water ballast.
It has a 15 HP honda which i believe is too big,
It had a std prop until last (Southern hemisphere) summer when i whacked the bottom. (Reversing into a beach with centre-board and rudder up, the prop was the lowest point.)
This event "spun" the prop in the rubber bush.
New prop was a Solas low pitch/ Hi thrust prop. No change in top end speed which is achieved on about 1/3 throttle, but it does feel like i can punch into wind and waves better than with original prop.
I achieve 6.8 knots in smooth water at 1/3 throttle and 7.0 at full throttle with lots extra noise and vibration.
I think 15 HP is more than i can use, but it seems that 4 stroke motors of ~10 HP are just de-tuned versions of 15 HP units and they weight the same.
Show me a 10 HP 4 stroke that is significantly lighter than my Honda 15 HP and i will buy it! (assuming not a cheap chinese brand of course!)
Cheers

 

[VicS]

I have a 26 ft light displacement trailer -sailor.
This has a planing hull and water ballast.
It has a 15 HP honda which i believe is too big,
, but it seems that 4 stroke motors of ~10 HP are just de-tuned versions of 15 HP units and they weight the same.
Show me a 10 HP 4 stroke that is significantly lighter than my Honda 15 HP and i will buy it! (assuming not a cheap chinese brand of course!)
Cheers

The 10 HP Honda is a smaller engine than the 15hp (222cc vs 350cc ) I think you will find although only about 4.5 kg lighter.

A 9.8 hp Tohatsu is about 9.5 kg lighter than the Honda 15 hp ... That's about 20% lighter

 

[ninky]

Sail drive propeller will give you better thrust (don't ask me to explain why though). Are you measuring speed through the water rather than SOG? If you're doing 5.4 over the ground into a Thames tide, you're doing well.

Nicola